JPH06230207A - Formation of dispersing - type bragg reflector in optical medium - Google Patents

Abstract

PURPOSE: To provide an improved method for forming a Bragg diffraction grating in a photosensitive light guide medium by exposing an interference pattern, formed by putting chemical ray beams on top of another. CONSTITUTION: When the Bragg diffraction grating is formed by conventional technology, the diffraction grating tends to show a reflection factor spectrum, having a series of subpeaks that are not desirable on the whole in addition to a center peak. These subpeaks are caused by interference effects of waves. This method corrects or removes the subpeaks by spatially varying the mean refractive index of the diffraction grating or spatially varying the diffraction grating cycles.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to the fabrication of optical waveguide components such as optical fibers, and more particularly to the formation of passive optical components integrated in optical waveguide components by using actinic radiation to alter the index of refraction. Regarding

[0002]

BACKGROUND OF THE INVENTION Certain optical media, including at least some silica-based optical fibers, can be altered in nature by irradiating them with electromagnetic waves in the proper spectral range. Below, such radiation (typically,
UV rays are called "chemical" rays. When a photosensitive optical fiber (or other optical medium) is exposed to actinic radiation, the refractive index of the exposed portion of the optical medium can change. For example, a pair of substantially monochromatic radiation beams emitted from a laser can be superimposed, for example by superimposing a periodic pattern on the incident light.

When a patterned field as described above is incident on an optical fiber or other optical waveguide having a suitable photosensitive core, the corresponding pattern is periodic (or quasi-periodic) in the core's refractive index. ) The core is loaded in the form of fluctuations. The above pattern (often referred to as a "Bragg grating" or "dispersive Bragg reflector (DBR)") can function as a spectrally selective reflector for electromagnetic waves. The Bragg diffraction grating formed by the above method is particularly useful as a terminal reflector of an optical fiber laser.
Bragg gratings are useful because they have spectral selectivity and are easily incorporated into the same optical fiber that supports the active laser medium.

Techniques for forming Bragg gratings are described in US Pat. Nos. 4,725,110 and 1989, issued to W. H. Glenn et al. On February 16, 1988. It is described in U.S. Pat. No. 4,807,950 issued February 28 to W. Et. Glenn et al. Fiber optic lasers with distributed flag reflector termination cavities are described in Optical Letter 17 (1992), pages 420-422, G.A. Ball and W. W. Morley (W. W. Morey), "Continuous Tunable Single-Mode Erbium Fiber Laser".

Bragg gratings are useful as passive optical components for other end reflector applications in fiber lasers. An optical filter consisting of a Bragg grating formed in an optical fiber is described in U.S. Pat. No. 5,007,705 issued to W. W. Moray et al. On April 16, 1991.

The inventors have found that when a pair of intersecting laser beams is used to form a Bragg grating in an optical filter, the grating can exhibit certain desirable optical properties as a whole. Are observing. Specifically, the reflectance spectrum of the diffraction grating has one or more relatively sharp sub-peaks or a regularly spaced row of these sub-peaks on one side of the central peak (generally on the short wavelength side). May be shown. These sub-peaks are called fine structure. This fine structure is not desirable, for example, in a feedback stabilization system where the output wavelength of the laser is fixed at the central peak of the Bragg grating. When the grating has sub-peaks, the wavelength tuning of the laser can shift towards the sub-peaks in response to environmental disturbances. Therefore, with a fine structure, such a laser system may be less robust against environmental disturbances.

For purposes of explanation, FIG. 1 shows experimental measurements of the transmittance spectrum of a typical Bragg grating formed in an optical fiber. Without loss, the sum of transmittance and reflectance is 100%. The transmittance spectrum contains a broad main peak and a series of minor peaks 15.

The inventors believe that the fine sidebands are due to interference effects related to the average axial profile of the refractive index within the diffraction grating region. The term "axial" direction means the direction of travel of electromagnetic waves in the diffraction grating. That is, the index of refraction of the fiber (or other waveguiding medium) in the grating region is typically the perturbation δ (Z), which represents the difference between this index and the index of the unexposed fiber, and the axial direction (ie, Z). Direction) perturbation variation. The perturbation varies periodically in step with the continuous light and dark stripes in the perturbed interference pattern.

However, each coherent beam has a spatially varying intensity profile in a plane perpendicular to the direction of travel of the beam. This profile is typically Gaussian. The intensity profile of the coherent beam defines the spatial spread of the Bragg grating,
Modulates the amplitude of the periodic index perturbation. As a result, the perturbation δ (Z) generally takes the form of a series of periodic peaks surrounded by an envelope (typically a Gaussian form). This envelope becomes maximum at or near the center of the diffraction grating and becomes 0 at the end of the diffraction grating. When the perturbations are averaged over an axial distance that is significantly larger than the grating period (eg, over 10 periods), the average perturbation will, of course, be of the same shape as the envelope.

The existence of such an envelope is well known. In fact, it is well known that side lobes can be produced in the reflection spectrum, for example an envelope having a rectangle (Bellsystem Technical Journal 5).
5 (1976), pp. 109-126, see, for example, H. Kogelnik's paper "Filter Response of Non-Uniform Approximate Periodic Structures". But,
The basic reason for this result is that the grating has a finite spatial extent. In comparison, the fine structure is the result of spatial mean perturbation. This mean perturbation is effective at certain points as a "dark" perturbation (which has a different physical meaning than the rapid change of the grating (ie, "line")). To date, no sufficient discussion of the effects of average perturbations on the spectral structure has appeared in the relevant technical literature. In particular, those skilled in the art have not paid attention to possible techniques for mitigating (or improving) the resulting microstructure.

[0011]

When a Bragg grating is formed according to the prior art, it tends to exhibit a reflectance spectrum with a central peak as well as an overall undesirable series of sub-peaks. . These sub-peaks are caused by the wave interference effect. The present invention provides an improved method of modifying or removing sub-peaks to form a Bragg grating in a photosensitive optical waveguide medium.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted an average refractive index perturbation δ ave by computer simulation.
When the (Z) profile takes a pulse-like form in some part of the Bragg grating, it has been successfully demonstrated that the grating can act as a resonant cavity for light of a certain wavelength. In the specification, the term "light" is used to refer to radiation in the ultraviolet, visible and infrared regions of the electromagnetic spectrum. Interference in the resonant cavity (similar to Fabry-Perot resonance) is the main cause of the sidebands (ie, microstructure) that we have observed in laboratory experiments. The inventors have discovered a method of softening (or improving) the above microstructure.

Accordingly, one embodiment of the invention relates to a method of forming a Bragg grating (also referred to as a distributed Bragg reflector) in a photosensitive optical medium. The dispersive flag reflector exhibits a reflectance spectrum that includes a main peak with a measurable amplitude. A distributed flag reflector consists of a material region of an object whose index of refraction is the sum of the initial index and at least the first (spatial or quasi-spatial) index perturbation. An example of a quasi-periodic diffraction grating is a grating whose period varies with position (ie, the so-called "linear chirp shape").
Diffraction grating). This sum has at least one vacuum Bragg wavelength (ie, the vacuum wavelength of the electromagnetic wave that satisfies the Bragg condition in at least a portion of the dispersive flag reflector) and, as a result, is relatively stronger than the dispersive flag reflector. Is reflected.

The method of the present invention comprises the step of producing two non-collinear beams of electromagnetic waves having actinic wavelengths. Each of these beams typically has a Gaussian intensity profile in cross section. More generally speaking, each beam radial direction has an intensity profile including at least one rising portion and at least one falling portion. The two beams are incident on at least a part of the medium, and the interference pattern is formed on the incident part, where the first perturbation occurs.

As a result of this actinic radiation exposure, the entrance portion takes the sum of the initial index of refraction and the first perturbation, and this sum is applied over at least 10 dispersed flag reflector periods or quasi-periods. It will have a dark refractive index defined by spatial averaging. Generally, the dark refractive index is
The beam intensity profile is partially changed depending on the position in the incident portion direction so as to have an ascending part and a descending part corresponding to the ascending part and the descending part. The vacuum Bragg wavelength at any point of the entrance is generally due to the dark index of refraction at that point.

The incident portion generally has a dark refractive index of the first
The first end and the second end that rise at the end and fall at the second end
And at least one section having an end (referred to as a "resonance section"). In each resonance section, if there is no refractive index other than the above-mentioned first perturbation, there is at least one electromagnetic wave vacuum wavelength satisfying the Bragg condition at the first end and the second end, and the first end and the second end. In the region between the edges, each property does not satisfy the Bragg condition. Generally, the resonant section gives a sub-peak to the reflection spectrum of the distributed flag reflector.

Compared to the prior art method, the method according to one embodiment of the invention is
A second index perturbation occurs in at least a portion of the entrance such that the sub-peak is modified. According to another embodiment of the present invention, sub-peaks are suppressed or improved.

Other embodiments of the present invention include steps that replace or add to the second perturbation step.
This embodiment includes varying the perturbation period with the axial position during the injection step so that the sub-peaks are suppressed or improved.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have developed a scanning interferometer having a structure in which the position of an interference pattern can be changed along a fiber while maintaining its alignment accuracy by parallel movement of a single movable mirror. We have found that it is advantageous to create an interference pattern using. As a result, the fiber can optionally remain fixed during each exposure step leading to the formation of multiple Bragg gratings.

An example of such an interferometer is shown in July 1978.
On the 6th day of the month, Zee Björklund (CC
Bjorklund) issued U.S. Pat. No. 4,093,33
No. 8. The optical configuration of this interferometer is shown in FIG. Such an optical configuration includes a laser light source 80, a translatable mirror 85, a rotatable mirror 120, and mirrors 90, 100 and 110. The coherent beam is focused on a photosensitive optical medium 130 (as an example, an optical fiber). The interference pattern can be repositioned along the fiber by translating the mirror 85 (without affecting its phase).

In general, the periodicity of the interference pattern can be changed by adjusting the crossing angle φ of the coherent beams. According to the interferometer of this example, the change of the periodicity is achieved by rotating the mirror 120 without changing the optical path length difference between the coherent beams. Two
An array of one or more diffraction gratings is easily formed in this optical fiber by translating the mirror 85 and then exposing a new portion of the fiber.

According to the preferred method of making a Bragg grating, the fiber is first clamped in place to ensure that the area to be exposed is straight. The fiber is exposed to effective radiation (typically ultraviolet). A variety of suitable UV sources are available and known to those of ordinary skill in the art.

To explain, the present inventors have found that the wavelength is about 245 nm.
It was discovered that an excimer-excited, frequency-doubled tunable dye laser, which emits at, is a suitable exposure source. The inventors of the present invention filed U.S. patent application pending on May 5, 1992 filed by D. J. DiGiovanni et al. On the use of such an exposure source. 07/878791.

As detailed in this US patent application, the exposure source is useful for fabricating diffraction gratings in heavily erbium-doped and silica-based optical fibers. These fibers are typically exposed to 2 mJ pulses at a repetition rate of 20 pulses per second. The cylindrical lens has a laser beam of 0.5 cm in length and 100 to 200 μm.
Focus on a band of m width. A typical exposure lasts about 30 seconds. By this method, the Bragg diffraction grating is easily formed with a constant period of about 0.5 μm.

The shape of the intensity profile of the laser exposure source described above has an approximate Gaussian distribution. As a result, the refractive index perturbations in the resulting Bragg grating appear as waves with a period modulated by the envelope of an approximate Gaussian distribution. Perturbations averaged over a number of periods, of course, have almost the same shape as the envelope. The appearance of waves with the above period is an example of an envelope that is at least partially pulsed.

The term "pulse shape" refers to as the axial position increases along a continuous section of the grating.
It means that the refractive index of the core takes a maximum value and then a low value.

The fine structure also has a negative pulse-shaped envelope (that is, an envelope having a minimum value, not a maximum value, near the center).
Is formed by a diffraction grating having. Such diffraction gratings are atypical, but these atypical diffraction gratings are also included in the definition of a diffraction grating having a pulse-shaped envelope.

The inventors have discovered, in a qualitative way, a simple conceptual model that helps explain the resonant behavior of pulse-shaped perturbations. This model will be described with reference to FIG. The average refractive index in the grating (ie,
The indices of refraction averaged over a large number (eg, 10) of grating periods are n ₁ , n ₂ and n ₃ (where approximately n ₁ = n ₃ , n ₁ <n ₂ > n ₃ ) refraction, respectively. It is represented as three rectangular pulses with a rate. The wavelength λ of the light traveling in each pulse is related to the corresponding vacuum wavelength λvac according to equation (1) below.

[0030]

[Equation 1]

In the equation, n is a relative refractive index.

For wavelengths near the center of the main reflectance peak of the diffraction grating, the Bragg condition is satisfied at the center of the average refractive index profile (ie at the second pulse). However, some wavelengths in the first pulse and the third pulse satisfy the Bragg condition, but do not satisfy the Bragg condition in the central pulse. As a result, electromagnetic waves with these wavelengths are
It is reflected in the pulse and the third pulse. On the other hand, in the second pulse, the electromagnetic wave travels freely and is not reflected. Also, at least a portion of this electromagnetic wave is trapped between the two reflective edge pulses. This creates a Fabry-Perot cavity. The reflectance spectrum of such a diffraction grating generally shows a fine structure present at a wavelength shorter than the center of the main reflectance peak. This fine structure peak corresponds to the standing wave in the Fabry-Perot cavity.

According to a more detailed analysis, the Bragg grating has a design wavelength λ 0. This design wavelength is the vacuum wavelength of the peak reflectivity of an ideal diffraction grating with a given period and an average index of refraction no equal to the average index of the unperturbed core material. As the intensity and duration of exposure of the grating to the coherent beam increases, the perturbation increases without changing the period of the grating. As a result, the peak vacuum wavelength is changed from .lambda.0 to another wavelength. λ0
Is given by the following formula.

[0034]

[Equation 2]

This detuning is generally towards longer vacuum wavelengths, as increasing exposure generally increases the index of refraction. Due to the intensity profile of the coherent beam, this effect generally varies with axial position within the diffraction grating.

The ideal diffraction grating is
It is reflective about the wavelength λ 0. In other words, the light of wavelength λ0 has an evanescent component over the entire length of the diffraction grating. Here, the diffraction grating is
Suppose there is no detuning, longer in the center than at the edges. In this case, the vacuum wavelength slightly longer or shorter than the wavelength λ 0 becomes evanescent only in the central portion of the diffraction grating where the diffraction grating becomes the strongest. This situation is shown in FIG.

The deviation of the given vacuum wavelength from the wavelength λ 0 is measured in the direction of the vertical axis labeled Δ in the figure. The axial coordinates are labeled Z. In the figure, a portion appearing as a shaded area is a locus of all points (Z, Δ) satisfying the Bragg condition. To determine which part of the grating is reflective for a given vacuum wavelength,
A horizontal line is drawn with the corresponding Δ value. The intersection of the shaded area and this horizon is projected on the Z axis. This projection defines the axial length of the reflector of the diffraction grating.

The effect of detuning is described in detail in connection with FIG. As usual, the detuning of the grating is assumed to be towards the longer vacuum wavelength. Now, since the diffraction grating is most effective for the Nagai wavelength rather than the wavelength λ0, the entire shadow area moves to the longer wavelength side due to detuning, and the main reflectance peak is
It moves from the wavelength λ0 to the longer vacuum wavelength λ1. However, when all coherent beams have a pulsed profile, the detuning is stronger in the center than at the ends of the grating. As a result, the central portion 2 of the shaded area
0 is moved more than both ends 25. As a result, in the diffraction grating, there may be a range of wavelengths shorter than λ1 that is reflected by the weak but less detuned ends, but not by the strong but detuned central part.

With respect to these wavelengths, the central part behaves like a resonant cavity, similar to the central pulse of the conceptual model described above. According to the same analogy, the reflective end 2
5 (hereinafter, referred to as “wing”) corresponds to two pulses that are both side boundaries of the central pulse. This situation is shown in FIG.

As mentioned above, the detuning of a portion of the grating can be increased by increasing the exposure of this portion to the index modifying radiation. As the detuning increases, the corresponding part of the shaded area in FIG. 4 moves to the long wavelength side. The resonant cavity does not reflect light anymore (if it does not, it resonates at the center of the diffraction grating), so that the resonant cavity has two wings 2
It is clear from the figure that it can be avoided by moving at least one of 5 to the long wavelength side. This movement can be easily performed by injecting the actinic ray beam from one incoherent beam into the diffraction grating. This index-correcting beam is displaced from the center of the diffraction grating. The microstructural changes resulting from such processing are easily observed and can be monitored during processing. The above process can be terminated when the microstructure is reduced to an acceptable level.

An example of a non-coherent refractive index modified exposure is shown in FIGS. The uncorrected diffraction grating has the spectrum shown in FIG. 1 (by computer simulation).
The resulting diffraction grating. This diffraction grating is 1557.4.
It has a design wavelength of 5 nm, a peak refractive index change of 0.08% due to actinic radiation and a full width at 1/2 of the maximum value of 1.75 nm. The diffraction grating is formed in a lossless optical fiber having a cladding index of 1.45.
FIG. 5 shows the refractive index profile of an unmodified diffraction grating.
The refractive index is expressed in terms of the corresponding vacuum Bragg wavelength, as described above. In the figure, curve 30 represents the spatially averaged refractive index, and curves 35 and 40 respectively represent
It represents the longest wavelength and the shortest wavelength reflected in the diffraction grating.

The refractive index correction exposure will be described with reference to FIG.
The actinic beam has a Gaussian distribution profile with the same spatial extent as the unmodified grating. The center of the index modifying beam is offset to a position where the profile shown in FIG. 5 drops by e ⁻² times its peak value. The beam profile (according to the simulation) shown in FIG. 7 is
It is expressed as a total refractive index profile (in terms of equivalent vacuum wavelength) produced by the exposure in the photosensitive medium in the initial state. FIG. 7 is a simulated refractive index profile of the same diffraction grating after exposure modification shown in FIG. In relation to the area between the curves 35 and 40, the right wing shown in FIG. 5 has been removed and the left wing has been reduced to a very narrow area with little or no contribution to the fines. it is obvious.

FIG. 8 shows the transmittance spectrum of the modified diffraction grating. It is clear that the resolved sub-peak 15 of FIG. 1 has been removed.

Refractive index modified exposures are not limited to exposures with a Gaussian profile. Various other spatially modulated exposures are easy to perform and in some cases may be desirable. For example, tilted exposure can be achieved by manipulating the grating region with a relatively small beam of actinic radiation. During scanning, the average intensity of the beam is varied, for example as a linear function of axial position. The average intensity of a continuous laser beam is easily modulated, for example by changing the input to the laser. The average intensity of the pulsed laser beam is easily modulated by changing the pulse repetition rate, for example.

As noted above, the microstructure is typically
It appears at a wavelength shorter than the center of the main reflectance peak of the diffraction grating. In some cases, it may be desirable to move the microstructure to a wavelength longer than the center of the main peak rather than removed. With reference to the simplified model of FIG. 2, the position of the microstructure can be reversed with respect to the main peak by reversing the first pulse and the third pulse with respect to the second pulse.
That is, one incoherent beam is selectively applied to the first pulse region and the third pulse region so that the average refractive index is larger in the first pulse region and the third pulse region than in the central region. It can be used to expose. The result is shown in FIG. As a result of this process, the vacuum wavelength that satisfies the Bragg condition is greater for the first and third pulses than for the second pulse. Therefore,
There is a longer wavelength than the center of the main reflection peak that reflects in the side pulse but not in the center pulse.

It should be noted that the refractive index modification exposure described above may reduce the intensity of the exposed portion of the diffraction grating.

Another method of modifying the index of refraction is to heat the diffraction grating. The inventors have discovered that in at least some photosensitive glasses, heating the Bragg grating tends to erase it (ie, reduce the amplitude of the index perturbation). Therefore, results similar to those described above can be achieved by using local heating to modulate the average refractive index of the grating. Such local heating is easily achieved by injecting a beam oscillated from an infrared laser such as a carbon dioxide laser into the photosensitive medium. The beam is arbitrarily offset along the photosensitive medium during exposure. Various other localized heating methods are possible, such as heating with a proximity tip of a heated needle-shaped probe.

A second way to remove, improve or reverse the microstructure is to change the period of the grating. This technique may be implemented in place of or in addition to the technique of changing the average index of refraction. According to the second technique, the vacuum wavelength satisfying the Bragg condition in a given region is lengthened by lengthening the period of the diffraction grating in this region and shortened by shortening the period of the diffraction grating. The fine structure is removed by lengthening the period of the grating on one side of the grating. this is,
For example, by forming a diffraction grating (that is, a linear chirp diffraction grating) whose period varies with axial position.

It is well known that a chirped grating can be formed in a photosensitive material by injecting a pair of coherent, non-collinear, focused actinic radiation beams into the photosensitive material. Since these beams are focused, the effective crossing angle is position dependent. The local period of the diffraction grating changes as the crossing angle changes. The effect of such a change in the spectral behavior of the grating is qualitatively similar to the effect of adding a linearly varying index perturbation. In fact, it will be appreciated by those skilled in the art that changing the grating period produces a number of effects similar to those produced by changing the average index of refraction.
it is obvious.

The official method of making a chirped grating involves the use of the interferometer shown in FIG.
It is described in a pending U.S. patent application entitled "Method of forming a spatially variable dispersive Bragg reflector in an optical medium" filed by V. Mizrahi et al. In summary, a tunable actinic radiation source is used and the actinic radiation wavelength is changed during exposure while the interference pattern is offset along the photosensitive medium. Also, the actinic wavelength is held constant and the crossing angle of the (plural) actinic beams is varied. This is accomplished by replacing one of the fixed plane mirrors of the interferometer optical system with a curved mirror and shifting the interference pattern during exposure.

In some cases, it is actually desirable to improve rather than soften the microstructure. For example,
The superposition of highly visible microstructures can result in a sharp central peak in the grating reflectance spectrum. It is desirable to have a very sharp spectral peak,
This is useful in applications where the presence of multiple peaks in close proximity is acceptable. The microstructure can be improved by additional detuning of selected central portions of the grating. Such additional detuning can be accomplished by injecting a single actinic radiation beam onto a selected portion of the diffraction grating. For example, the grating can be exposed with an axisymmetric beam having a Gaussian distribution profile in the same spatial range as the unmodified grating. The simulated transmittance spectrum of the diffraction grating of FIG. 5 after such exposure is shown in FIG. As is apparent from the above description, the same effect can be achieved by spatially changing the diffraction grating period.

In some cases, the fine structure of the uncorrected diffraction grating is characterized by roughness of the main peak by superimposing one or more well resolved subpeaks on a slowly varying background provided by the side of the main peak. to add. Such a situation is schematically shown in FIG. In FIG.
Sub-peak 45 (completely resolved and shown as curve 50) adds roughness to the spectrum. On the other hand, the sub-peak 55 (completely resolved and shown as curve 60) is
It only widens the main peak without substantially roughening it. For example, when the laser is fixed on the central peak of a Bragg grating by feedback stabilization, the roughness may cause it to increase the ambiguity around the peak position, causing the amplifier tuning to jump to the subpeak. I don't know, so it's not desirable.

Therefore, one application of the present invention is to reduce the roughness added by the microstructure. One way to quantify the roughness is to express it in terms of the height of the sub-peak above the slowly varying background. This height is preferably normalized to the height of the main peak. Thus, the roughness corresponding to feature 45 shown in FIG. 11 is such that the maximum roughness added by the peak is less than 10% for purposes such as amplifier tuning expressed as the ratio of Hs to HM. It is desirable to be corrected to.

[0054]

According to the present invention, when the Bragg diffraction grating is formed in the photosensitive optical waveguide medium, it is caused by the interference effect of the waves of the plurality of actinic radiation beams used for forming the interference pattern. An improved or modified method of the series of subpeaks is obtained. When a Bragg grating is formed according to the prior art, it tends to exhibit a reflectance spectrum with a series of unwanted peaks in addition to the central peak. These sub-peaks are modified or removed by spatially changing the average refractive index of the grating or by spatially changing the grating period.

[Brief description of drawings]

FIG. 1 is a computer-simulated transmittance spectrum of a typical Bragg grating formed in an optical fiber.

FIG. 2 is a schematic showing the spatial variation of the average refractive index of a typical Bragg grating formed in an optical medium.

FIG. 3 is a schematic diagram showing how interference effects can occur in a Bragg grating having a pulsed average refractive index profile.

FIG. 4 is a schematic diagram showing another mechanism by which interference effects can occur in a Bragg grating having a pulsed average refractive index profile.

FIG. 5 is a refractive index profile obtained by computer simulation of the Bragg diffraction grating shown in FIG.
In FIG. 5, as in FIGS. 6 and 7, the average refractive index perturbation is expressed in terms of vacuum wavelength. At a given point, the upper and lower limits on the vacuum wavelength where the diffraction grating is a reflector are shown.

FIG. 6 is a computer-simulated profile of refractive index perturbations performed by an exemplary refractive index modified exposure, in accordance with one embodiment of the present invention.

7 is a computer simulated refractive index profile of the Bragg grating shown in FIG. 5 after the refractive index modification of FIG. 6 has been made.

8 is a computer-simulated transmittance spectrum of the modified Bragg grating shown in FIG. 7. FIG.

9 is a schematic diagram showing a modification of the interference effect shown in FIG. 4, in which the resulting sidebands are reversed with respect to the central reflectance peak. According to one embodiment of the invention, this modification is achieved by changing the average refractive index.

FIG. 10 is a transmission spectrum diagram by computer simulation of the Bragg grating shown in FIG. 5 after a refractive index correction exposure for improving a microstructure according to an embodiment of the present invention.

FIG. 11 is a reflectance spectrum diagram of a virtual Bragg diffraction grating showing the tendency of some sub-peaks to widen the width of the main peak and the tendency of other sub-peaks to add roughness to the side of the main peak. .

FIG. 12 is a schematic diagram showing the optical arrangement of an example of a prior art interferometer useful for carrying out the method of the present invention.

[Explanation of symbols]

 20 Central part 25 End part (wing)

Front Page Continuation (72) Inventor Victor Misrahi America, 07921, New Jersey, Bedminster, Cardinal Lane 412 (72) Inventor John Edward Zaip Canada, M5B2H9, Toronto, Apt. 1217, Carlton Street 45

Claims (12)

[Claims] 1. A light-transmissive photosensitive object having at least one light traveling axis, wherein a Bragg condition is defined in the object and has a main peak for a vacuum wavelength of light that approximately satisfies the Bragg condition. In the method of manufacturing an article in which a first refractive index perturbation exhibiting a reflectance spectrum is formed, (a) a step of generating two actinic ray beams, each actinic ray beam having at least one actinic ray beam in a cross section. Actinic beam generation step having an intensity profile including an ascending part and at least one descending part, and (b) an interference pattern is formed in the light traveling axis direction to reach the formation of a refractive index perturbation. As described above, since the step of injecting the actinic ray beam into at least a part of the photosensitive object and the formation of the first refractive index perturbation are reached, an interference pattern is formed in the light traveling axis direction. In the steps of incident actinic beams on at least a portion of said photosensitive body, c. The step (b) leads to a spatially averaged refractive index profile in the first perturbation having at least one rising portion and at least one falling portion in the light traveling axis direction, and the “resonance section” At least one section, designated as, is included between the rising portion and the falling portion, so that the reflection spectrum without exposure to actinic radiation includes at least one subpeak. And (d) before or after the step (b), a step of causing a change in relative amplitude between the main peak and the sub-peak by causing a second refractive index perturbation in the object. A method of forming a distributed Bragg reflector in an optical medium. 2. The method of claim 1, wherein step (d) includes the step of injecting a single incoherent actinic radiation beam onto at least a portion of the photosensitive object. 3. A step of displacing the one actinic ray beam in the light traveling axis direction during the step of injecting the one actinic ray beam, and a step of changing an average intensity of the one actinic ray beam. The method of claim 2, further comprising: 4. The method of claim 1, wherein step (d) comprises heating at least a portion of the photosensitive object. 5. The method of claim 4, wherein the heating step includes the step of directing a beam from an infrared laser to at least a portion of the photosensitive object. 6. The method according to claim 1, wherein the step (d) increases the amplitude of the sub-peak with respect to the main peak. 7. The method according to claim 1, wherein the step (d) increases the amplitude of the sub-peak with respect to the main peak. 8. The method according to claim 1, wherein the step (d) increases the amplitude of the sub-peak with respect to the main peak. 9. The first perturbation has a midpoint, and the first
3. The method of claim 2, wherein the actinic ray injection step comprises injecting the actinic ray beam onto a portion of an object axially offset from the midpoint. 10. The method of claim 1, wherein the step (d) substantially removes the resonant section. 11. In the absence of the second perturbation, the sub-peak is overlaid on the main peak side so that the spectrum has roughness relative to the main peak side, and step (d) is A method according to claim 1, characterized in that, due to these substeps, the roughness is reduced by less than 10% relative to the main peak amplitude. 12. A light-transmissive photosensitive object having at least one light traveling axis, wherein a Bragg condition is defined in the object and has a main peak for a vacuum wavelength of light that approximately satisfies the Bragg condition. In the method of manufacturing an article in which a first refractive index perturbation exhibiting a reflectance spectrum is formed, (a) a step of generating two actinic ray beams, each actinic ray beam having at least one actinic ray beam in a cross section. Actinic beam generation step having an intensity profile including an ascending part and at least one descending part, and (b) an interference pattern is formed in the light traveling axis direction to reach the formation of a refractive index perturbation. As described above, an interference pattern is formed in the light traveling axis direction in order to reach the step of injecting the actinic ray beam into at least a part of the photosensitive object and the formation of the first refractive index perturbation. Sea urchin, a step of entering the actinic beams on at least a portion of said photosensitive body, c. The step (b) leads to a spatially averaged refractive index profile in the first perturbation having at least one rising portion and at least one falling portion in the light traveling axis direction, and the “resonance section” At least one section labeled d is included between the ascending section and the descending section, d. During the step (b), in the ascending section and the descending section, a cycle is set according to the axial position so that all the vacuum wavelengths satisfying the Bragg condition satisfy the Bragg condition even in a considerable part of the resonance section. A method of forming a distributed Bragg reflector in an optical medium, characterized in that it is varied to produce a reflectance spectrum that is substantially free of subpeaks.